Fix a bug in OptimizedStructLayout when filling gaps before

fixed fields with highly-aligned flexible fields.

The code was not considering the possibility that aligning
the current offset to the alignment of a queue might push
us past the end of the gap.  Subtracting the offsets to
figure out the maximum field size for the gap then overflowed,
making us think that we had nearly unbounded space to fill.

Fixes PR 51131.